And because AC is equally divided

According to the triangle principle, if two angles are equal, then two conditional edges are equal.

Then BC=AB

Similarly:. AD=AB。

So: AB=AD=BC

Because AE//BF, and then AD//BC.

According to the diamond judgment rule: three sides of a quadrilateral are equal and two adjacent sides are parallel.

It can be confirmed that ABCD is a diamond.

The second train of thought:

Because AE//BF, then

And because AC is equally divided

According to the triangle principle, if two angles are equal, then two conditional edges are equal.

Then BC=AB

The same goes for the front.

Because BD is divided equally

So < ADB = & ltDBC< associate doctor.

Another reason

So < AOB = & ltAOD=90 degrees.

Namely: AC vertical BD (it has been proved above that BC=AB)

According to the diamond judgment rule: two diagonal lines of a quadrilateral are vertical and two adjacent sides are equal, it can be proved that this quadrilateral is a diamond.